1. Field of the Invention
The present invention relates generally to fabrication of metal oxide semiconductor field effect transistors (MOSFETs), and, more particularly, to MOSFETs that achieve improved carrier mobility through the incorporation of strained silicon.
2. Related technology
MOSFETs are a common component of integrated circuits (ICs). FIG. 1 shows a conventional MOSFET device. The MOSFET is fabricated on a semiconductor substrate 10 within an active area bounded by shallow trench isolations 12 that electrically isolate the active area of the MOSFET from other IC components fabricated on the substrate 10.
The MOSFET is comprised of a gate electrode 14 that is separated from a channel region 16 in the substrate 10 by a thin first gate insulator 18 such as silicon oxide or oxide-nitride-oxide (ONO). To minimize the resistance of the gate 14, the gate 14 is typically formed of a doped semiconductor material such as polysilicon.
The source and drain of the MOSFET are provided as deep source and drain regions 20 formed on opposing sides of the gate 14. Source and drain suicides 22 are formed on the source and drain regions 20 and are comprised of a compound comprising the substrate semiconductor material and a metal such as cobalt (Co) or nickel (Ni) to reduce contact resistance to the source and drain regions 20. The source and drain regions 20 are formed deeply enough to extend beyond the depth to which the source and drain silicides 22 are formed. The source and drain regions 20 are implanted subsequent to the formation of a spacer 24 around the gate 14 which serves as an implantation mask to define the lateral position of the source and drain regions 20 relative to the channel region 16 beneath the gate.
The gate 14 likewise has a suicide 26 formed on its upper surface. The gate structure comprising a polysilicon material and an overlying silicide is sometimes referred to as a polycide gate.
The source and drain of the MOSFET further comprise shallow source and drain extensions 28. As dimensions of the MOSFET are reduced, short channel effects resulting from the small distance between the source and drain cause degradation of MOSFET performance. The use of shallow source and drain extensions 28 rather than deep source and drain regions near the ends of the channel 16 helps to reduce short channel effects. The shallow source and drain extensions are implanted prior to the formation of the spacer 24 and after the formation of a thin spacer 30, and the gate 14 and thin spacer 30 act as an implantation mask to define the lateral position of the shallow source and drain extensions 28 relative to the channel region 16. Diffusion during subsequent annealing causes the source and drain extensions 28 to extend slightly beneath the gate 14.
One option for increasing the performance of MOSFETs is to enhance the carrier mobility of silicon so as to reduce resistance and power consumption and to increase drive current, frequency response and operating speed. A method of enhancing carrier mobility that has become a focus of recent attention is the use of silicon material to which a tensile strain is applied. xe2x80x9cStrainedxe2x80x9d silicon may be formed by growing a layer of silicon on a silicon germanium substrate. The silicon germanium lattice is generally more widely spaced than a pure silicon lattice as a result of the presence of the larger germanium atoms in the lattice. Because the atoms of the silicon lattice align with the more widely spread silicon germanium lattice, a tensile strain is created in the silicon layer. The silicon atoms are essentially pulled apart from one another. The amount of tensile strain applied to the silicon lattice increases with the proportion of germanium in the silicon germanium lattice.
Relaxed silicon has six equal valence bands. The application of tensile strain to the silicon lattice causes four of the valence bands to increase in energy and two of the valence bands to decrease in energy. As a result of quantum effects, electrons effectively weigh 30 percent less when passing through the lower energy bands. Thus the lower energy bands offer less resistance to electron flow. In addition, electrons encounter less vibrational energy from the nucleus of the silicon atom, which causes them to scatter at a rate of 500 to 1000 times less than in relaxed silicon. As a result, carrier mobility is dramatically increased in strained silicon as compared to relaxed silicon, offering a potential increase in mobility of 80% or more for electrons and 20% or more for holes. The increase in mobility has been found to persist for current fields of up to 1.5 megavolts/centimeter. These factors are believed to enable a device speed increase of 35% without further reduction of device size, or a 25% reduction in power consumption without a reduction in performance.
An example of a MOSFET using a strained silicon layer is shown in FIG. 2. The MOSFET is fabricated on a substrate comprising a silicon germanium layer 32 on which is formed an epitaxial layer of strained silicon 34. The MOSFET uses conventional MOSFET structures including deep source and drain regions 20, shallow source and drain extensions 28, a gate oxide layer 18, a gate 14 surrounded by spacers 30, 24, silicide source and drain contacts 22, a silicide gate contact 26, and shallow trench isolations 12. The channel region 16 of the MOSFET includes the strained silicon material, which provides enhanced carrier mobility between the source and drain.
One detrimental property of strained silicon MOSFETs of the type shown in FIG. 2 is that the band gap of silicon germanium is lower than that of silicon. In other words, the amount of energy required to move an electron into the conduction band is lower on average in a silicon germanium lattice than in a silicon lattice. As a result, the junction leakage in devices having their source and drain regions formed in silicon germanium is greater than in comparable devices having their source and drain regions formed in silicon.
Another detrimental property of strained silicon MOSFETs of the type shown in FIG. 2 is that the dielectric constant of silicon germanium is higher than that of silicon. As a result, MOSFETs incorporating silicon germanium exhibit higher parasitic capacitance, which increases device power consumption and decreases driving current and frequency response.
Therefore the advantages achieved by incorporating strained silicon into MOSFET designs are partly offset by the disadvantages resulting from the use of a silicon germanium substrate.
It is an object of the present invention to provide a strained silicon MOSFET device that exploits the benefits of strained silicon while reducing the detrimental effects of the use of silicon germanium to support the strained silicon layer.
In accordance with embodiments of the invention, a MOSFET incorporates a strained silicon layer that is supported by a silicon germanium layer. Strained silicon and silicon germanium at the locations of deep source and drain regions are removed and replaced with silicon regions. Deep source and drain regions are then implanted in the silicon regions. The formation of source and drain regions in the silicon regions reduces junction leakage and parasitic capacitance and therefore improves device performance compared to the conventional strained silicon MOSFET.
In accordance with one embodiment of the invention, a MOSFET incorporating strained silicon is fabricated. Initially a substrate is provided. The substrate includes a layer of silicon germanium having a layer of strained silicon formed thereon. The substrate further includes a gate insulator formed on the strained silicon layer and a gate formed on the gate insulator, and shallow source and drain extensions formed at opposing sides of the gate. A spacer is then formed around the gate and gate insulator. The strained silicon layer and silicon germanium layer are then etched to form trenches adjacent to the spacer at the opposing sides of the gate. Silicon regions are then formed in the trenches, and deep source and drain regions are formed in the silicon regions at the opposing sides of the gate. The depth of the deep source and drain regions does not extend beyond the depth of the silicon regions.
In accordance with another embodiment of the invention, a MOSFET incorporating strained silicon is provided. The MOSFET includes a substrate comprising a layer of silicon germanium, and a gate that overlies a strained silicon layer formed on the silicon germanium layer and that is separated from the strained silicon layer by a gate insulator. Silicon regions are formed at opposing ends of the gate adjacent to ends of the strained silicon layer. Shallow source and drain extensions are formed at opposing ends of the gate in the strained silicon layer, and deep source and drain regions are formed at the opposing ends of the gate in the silicon regions. The depth of the deep source and drain regions does not extend beyond the depth of the silicon regions.